Processes for producing consolidated metallurgical articles from metal powders are well known. For example, U.S. Pat. No. 4,747,999 to Hasselstrom describes a method of producing a consolidated powder metallurgical article using a particulate pressure medium which is preheated in a special container, in a "fluidized" bed. The bed is then heated to a forging temperature and transferred to an outer mold. Pressure is then applied to the pressure medium by means of a pressure tool. This method disadvantageously requires a pressure medium, a fluidized bed, and complex equipment to preheat the pressure medium and to transfer its contents to an outer mold.
Further, the method of the '999 patent does not provide a means to control shrinkage or physical dimensions in the ceramic mold and/or consolidated article, but rather, requires a shrinkage compensated model. This process does not allow existing parts, dies, molds and patterns with finished dimensions to be candidates for the original model, but rather, requires an oversized model, which increases the number of steps required and labor costs.
Further, the method of the '999 patent suffers from interparticle friction of the pressure medium. Interparticle friction is described in UK Patent 2140825A, in which a method is described to reduce such friction and provide uniform pressure. Interparticle friction requires increased pressure or heat to consolidate the powder metal article, requiring large equipment and energy costs. Further, the method of the '999 patent requires crushing of the ceramic mold to remove finished articles, and does not allow the ceramic mold material to be removed from internal passageways or cavities, thereby severely limiting part configuration possibilities.
Other prior art techniques use heat and pressure to increase further the density of a pre-consolidated powder metal article, requiring metal dies machined to desired part geometry. Such a die is loaded with metal powder and compressed in the range of 20,000-100,000 psi at room temperature. The article is then ejected from the die and transferred to a furnace and sintered to increase density and particle bonding. The article is then transferred to a forging die and is surrounded by a particulate pressure medium, wherein heat and applied pressure transferred through the medium further consolidates the metal article to high density.
Such prior art techniques require numerous steps and equipment to produce a powder metal article having complex shape, high density and dimensional accuracy. Cold compaction pressures of 20,000-100,000 psi, followed by sintering requires expensive powder compacting equipment, metal tooling and a sintering furnace to produce a pre-consolidated metal part. The additional steps of placing a pressure medium in a forging die, positioning the metal article upon this medium, covering the metal article with additional medium, and heating the pressure medium and metal article to a forging temperature to densify the metal article increases labor, material and energy costs.
An additional drawback of requiring a pressure medium is interparticle friction, as noted above, which can additionally cause distortion and loss of dimensional accuracy. See U.S. Pat. Nos. 4,539,175 and 4,501,718. The loss of dimensional accuracy requires secondary machining of the article to its final dimensions which increases costs. Further, the pressure medium has additional disadvantages of increasing surface area and volume, requiring larger forging dies, and requiring additional pressure and/or heat. Accordingly, use of a pressure medium requires larger equipment to consolidate a metal article, resulting in an increase in equipment, energy and labor costs.
U.S. Pat. No. 4,041,123 discloses a method of producing a powder article which requires producing a pre-consolidated article by mixing a ceramic powder and water to form a slurry which is casted into a porous mold. This pre-formed body has a void content of 30-60%. A pressure medium, heat and applied pressure are used to consolidate further the article to higher density. This method has several disadvantages. Slip casting requires a porous mold to be fabricated from an original article or rubber mold and requires a water content of 40-70% (leaving a particulate density of only 30-60% by weight). Due to the high water content, tremendous shrinkage results both when drying and in final consolidation. In addition, the density of a slip cast part varies, caused by larger, heavier powder particles settling at the bottom of the mold causing density gradients and resulting in non-uniform shrinkage. This distortion is further increased by the use of a particulate pressure medium. Interparticle friction of a pressure medium causes non-uniform pressure on the ceramic article being consolidated, resulting in distortion or loss of dimensional accuracy as noted in U.S. Pat. Nos. 4,501,718 and 4,539,175. This distortion or loss of dimensional accuracy increases the need for machining the article to its final dimensions and increases costs. The pressure medium increases the surface area or volume, thereby requiring higher pressure and/or heat for consolidation. Larger dies are necessary to accommodate the pressure medium resulting in increased machine size, labor and energy costs.
Other prior art techniques are shown in U.S. Pat. No. 4,041,123, which teaches a casted ceramic article that is made more dense by a pressure medium that requires heat and applied pressure to further increase density to the casted ceramic article. This technique can not produce a complex metal article in situ, but uses the heat and pressure operation only to make a further dense article.
The prior art method shown in U.S. Pat. No. 4,547,337 discloses a method to consolidate a powder in which the powder to be consolidated is placed in a hermetically sealed cylindrical container which is evacuated. The container is embedded in a glass material that becomes viscous at a desired temperature. Applied pressure deforms the glass pressure medium in turn applying pressure to the inner cylindrical container containing a powder which is then consolidated. It is mentioned that if a more intricately shaped article is required, the cylindrical inner container may be eliminated, and other materials such as elastomers could be used to produce an intricately shaped rubber mold to encapsulate the powder metal. The mold transfers its shape to the powder metal under pressure to produce an intricately shaped article. This method of manufacturing requires a pressure medium that collapses, deforms or becomes viscous under heat and/or pressure to consolidate a powder metal. The nature of a pressure medium which becomes viscous or deforms under heat and/or pressure would transfer this deformation to the loose powder metal during consolidation, causing distortion and loss of dimensional accuracy, thus requiring machining of the article to its final dimensions which increases costs. Additional steps of manufacturing a cylindrical container and then requiring a glass material to be casted around this container also increases costs.
The use of the pressure medium step has the additional disadvantage of requiring a larger forging die to contain this medium, and, in turn, increases surface area or volume which requires higher heat and/or pressure to consolidate the metal powder. It also increases machine size, energy and labor costs. This prior art mentions the use of the elastomer tooling to apply pressure to the powder metal. The method has the disadvantage of deforming the loose metal powder at room temperature and would require additional steps to place the article in a pressure medium using heat and applied pressure for further consolidation to produce a high density article. The article experiences further deformation by this additional consolidation step requiring secondary machining to its final dimensions which would increase costs.
A further process is shown in U.S. Pat. No. 4,389,362 which discloses a process for making a metal billet by encapsulating metal powder in a metal capsule, or as it is more commonly known a "metal can," and placing a pressure transmitting medium that becomes viscous at consolidation temperature between a first can and a second can, which must be fabricated to encapsulate this pressure medium. A second pressure medium is required to compress the second can. Heat and an applied pressure medium makes the second can more dense. The viscous pressure medium in turn makes the first can more dense, which contains the powder metal. This is a method that requires numerous troublesome steps to consolidate a metal powder. First, a metal can for housing the metal powder must be fabricated, which is usually done by sheet metal equipment. This first can requires embedding in an outer pressure medium, requiring another can to be manufactured and another pressure medium. This method requires two "cans" and two different pressure mediums, such as glass and talc. This method cannot produce a complex shaped article in situ but requires multiple manufacturing steps of producing metal cans to contain either a pressure medium or metal powder. The pressure medium that deforms or becomes viscous transfers that shape or deformity to the loose metal powder being consolidated. The additional outer pressure medium suffers from interparticle friction which causes uneven pressure and causes additional distortion which is disclosed in U.S. Pat. Nos. 4,501,718 and 4,539,175. The distortion causes loss of dimensional accuracy and requires machining of the article to its final dimensions which increases the steps and costs. These two pressure mediums increase volume and surface area which require larger dies, higher heat and/or pressure resulting in larger machinery requirements and higher energy and labor costs.